Method of forming electric insulating coating on the surface of silicon steel sheet with serpentine

ABSTRACT

A method of treating silicon steel sheet and forming an electric insulating coating having excellent adherent, heat resisting and electric insulating properties on the surface of a silicon steel sheet is disclosed comprising the steps of coating the surface of a silicon steel sheet containing 2 to 4 percent by weight of silicon with a separator formed from an aqueous slurry state consisting of 5 to 40 percent by weight of serpentine material and the remainder of magnesia to which may be added 4 to 40 percent by weight of manganous oxide powder, the coating will include the serpentine material in its dried state to an amount of 0.5 g/m2 to 6.0 g/m2, the magnesia in its dried state to an amount of 2 g/m2 to 12 g/m2 and the manganous oxide, if present, in its dried state in an amount of 0.5 g/m2 to 6.0 g/m2, the coating step is followed by drying the silicon steel sheet thus coated, and box annealing the coated and dried steel sheet in a reducing atmosphere containing hydrogen at a high temperature.

United States Patent [191 Hamachi et al.

[ METHOD OF FORMING ELECTRIC INSULATING COATING ON THE SURFACE OF SILICON STEEL SHEET WITH SERPENTINE [75] Inventors: Kazuo Hamachi, Nishinomiya;

Hiroshi Shimanaka, Chiba; Tatsuo Kawkkami, Takarazuka; Toshio Irie, Chiba; Akira Komoda, Tokyo, all of Japan [73] Assignee: Kawasaki Steel Corporation, Kobe City, Japan [22] Filed: Dec. 14, I970 [21] Appl. No.: 97,514

[30] Foreign Application Priority Data Dec. 18, 1969 Japan 44/101511 [52] U.S. Cl 148/113, l48/31.5, 148/122, 1 17/129 [51] Int. Cl. H011 1/04 [58] Field of Search 148/6, 6.14, 113, l48/31.5, 31.55, 27,122,13.1,12.1,l4, 28; 117/127, 129

[5 6] References Cited UNITED STATES PATENTS 2,354,123 7/1944 I-Iorstman et al. 117/129 X 3,189,483 6/1965 Trigg et al. 148/113 X 3,522,108 7/1970 Yamamoto et al. 117/129 X 3,364,057 9/1964 Jackson 117/127 X 3,151,000 9/1964 Schmidt et a1 148/113 X [451 Oct. 16, 1973 2,802,761 8/1957 Fast 148/113 3,583,887 6/1971 Steger et a1 148/113 3,084,081 4/1963 Carpenter 148/113 X 3,627,594 12/1971 Yamamoto et al. 148/113 OTHER PUBLICATIONS Hurlbut, C. S.; Danas Manual of Minerology, New York, 1941, pp. 305-7. Merriman, A. D., A Dictionary of Metallurgy, MacDonald and Evans, Ltd., London, 1958, Pages 188, 314 and 315 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. R. Satterfield Attorney-Robert E. Burns and Emmanuel J. Lobato [57] ABSTRACT A method of treating silicon steel sheet and forming an electric insulating coating having excellent adher of manganous oxide powder, the coating will include the serpentine material in its dried-state to an amount of 0.5 g/m to 6.0 g/m", the magnesia in its dried state to an amount of 2 g/m to 12 g/m and the manganous oxide, if present, in its dried state in an amount of 0.5 g/m to 6.0 glm the coating step is followed by drying the silicon steel sheet thus coated, and box annealing the coated and dried steel sheet in a reducing atmosphere containing hydrogen at a high temperature.

5 Claims, 10 Drawing Figures PATENTED BUT I 6 I975 SHEET 1 0F 5 FIG In FIG. lb

Temperature x |o0c EE Q O q 1 q q I I |23 6 789M Q. 5 1 5 x. 005m: :23 309600 6 EzoE PATENTEU EU 16 I873 SHEET 2 0F 5 oucocu l voucuco 25850 2885 5 zmama awrmoaww z. v28; :23 386mm 3 59:

PATENTEDUCT l 6 ma 3.765957 SHEET 5 BF 5 FIG. 5

O Separator according L2 to the invention R Y Y mg No Deterioration (Assumed) es I x 138 23: 310 2 gcOS n J6 l is 5v as lb u |.'2 (3 L4 is Iron Loss before the stress relief annealing treatment r,

X w a, Silicon steel sheet treated by the 9.0 separator according to the invention b. Silicon steel sheet treated by the separator b consisting solely of M90 METHOD OF FORMING ELECTRIC INSULATING COATING ON THE SURFACE OF SILICON STEEL SHEET WITH SERPENTINE FIELD OF THE INVENTION This invention relates to methods of forming electric insulating coating on the surface of silicon steel sheet and more particularly an improved method of forming an electric insulating coating having excellent adherent, heat resisting and electric insulating properties on the surface of a silicon steel sheet in an easy and uniform manner.

In general, a silicon steel sheet is required not only to have excellent magnetic and mechanical properties but also to be coated on the surface thereof with a thin uniform film having excellent adherent, heat resisting and electric insulating properties.

The principal object of the invention is to form a thin uniform glass film having excellent adherent, heat resisting and electric insulating properties on the surface of the silicon steel sheet, particularly throughout the surface of a wide silicon steel sheet or strip.

BACKGROUND OF THE INVENTION As a method of forming an electric insulating coating on the surface of a silicon steel sheet, it has heretofore been well known to coat the silicon steel sheet at a time when it is subjected to a high temperature box annealing treatment with a separator consisting of a refractory oxide powder such as magnesia so as to prevent adjacent silicon steel sheets in the form of a coiled or laminated body from sticking or welding to each other and cause the magnesia to react with the surface oxide of the silicon steel sheet during the high temperature box annealing treatment thus forming an electric insulating coating thereon. Moreover, a number of methods of improving the properties of the electric insulating coating have been proposed.

But, when carrying out such methods in an industrial scale it becomes more difficult to form an electric insulating coating having excellent properties in an easy and uniform manner throughout the surface of a wide steel or strip. In the prior arts a method has also been proposed comprising the steps of subjecting a continuous open annealing treatment to a silicon steel sheet in a hydrogen atmosphere containing water to oxidize silicon in the steel sheet and deposit silicon dioxide on the surface of the silicon steel sheet, coating the surface of the silicon steel sheet with a separator mainly consisting of magnesia, and then box annealing the coated silicon steel sheet, whereby to combine silicon dioxide with magnesia and form a glass film on the surface of the silicon steel sheet.

The disadvantage of the aforesaid prior art method consists in that since the effective thickness of the glass film obtained is determined by the amount of silicon dioxide deposited on the surface of the silicon steel sheet during the continuous open annealing treatment, the annealing conditions, for example, the surface condition of the silicon steel sheet prior to the annealing treatment, the annealing time and the temperature distribution in the furnace must accurately be controlled, and that, more particularly, there occurs decarburization and oxidation of silicon during the continuous open annealing treatment so that water vapour in the atmosphere is consumed with the result that water vapour must always be supplied in order to supplement the water consumed, thus rendering it difficult to control the atmosphere in the furnace and hence difficult to obtain a silica film having a desired thickness in a stable manner.

In general, the separator mainly consisting of magnesia is suspended in water to produce an aqueous suspension which is then coated on the surface of the steel sheet. If use is made of light magnesia having an adhesive force which is sufficient to prevent the film formed and dried on the surface of the steel sheet from flaking off therefrom, the hydration reaction of the light magnesia proceeds during its suspension in water to pro duce magnesium hydroxide which decomposes during the high temperature box annealing treatment to produce an excessive amount of water vapour thus preventing formation of a thin glass film having excellent properties.

In order to avoid such a disadvantage, it has been proposed to maintain the annealing temperature on the order of the decomposition temperature of magnesium hydroxide (about 400C) for the purpose of preventing production of the excessive amount of water vapour. This prior art method, however, not only wastes time but also makes it extremely difficult in practical operation in a uniform manner to decrease the water content remained in the space between adjacent wide silicon steel sheets in the form of a coiled or laminated body.

The above mentioned difficulty generally occurs if the separator contains hydrated oxides such as magnesium oxide or calcium oxide. Various other methods have also been proposed to avoid such difficulty, but these methods were insufficient to avoid it. For instance, in order to suppress the progress of hydration of magnesia, it has been proposed to calcine magnesia at a relatively high temperature and then suspend the calcined magnesia in water at a low temperature and coat the surface of the silicon steel sheet with the suspension thus obtained within a few minutes. Such method had been disclosed in U.S. Pat. No. 2,906,645.

But, the above mentioned steps have seldom been utilized in the industry for several reasons. That is, a surplus of the separator slurry obtained in the coating step must be discarded or in order to reproduce and reuse such surplus separator, it is necessary to effect a troublesome calcining step thus making the operating cost excessively high.

It has also been proposed to calcine alumina fine power at a high temperature and then coat the surface of the powder with calcium oxide to produce a refractory oxide powder which is then suspended in water to obtain a separator to be coated on the surface of the silicon steel sheet. Such method has the disadvantage that an xtra step is required in order to coat the calcined alumina fine powder with the calcium oxide.

It has also been proposed to use an organic solvent as the separator or to spray the separator powder, charged with static electricity, on the'surface of the silicon steel sheet. The former method is objectionable owing to the use of expensive organic solvent, while the latter method is unpractical as it involves an excessively high installation cost.

The inventors have carefully investigated the cause of producing glass films having inferior properties owing to the presence of the bound water of magnesia which has been the worst trouble arising in the case where water is utilized as medium for the separator and have noticed the following facts.

That is, of the separator contains large amounts of bound water, this bound water is delivered the separator during the temperature rise of the box annealing treatment. The bound water thus delivered oxidizes the surface of the steel sheet to produce iron oxide.

The inventors have found out that a direct cause of producing the glass films having inferior properties is not due to the presence of the iron oxide produced, but due to the reduction of the iron oxide by hydrogen arising in the case where the water vapour in the atmosphere is exhausted by the oxidation of the surface of the steel sheet.

All of the prior art methods have contemplated to merely restrict the amount of the bound water of magnesia which constitutes the cause of oxidizing the steel sheet.

THE INVENTION As distinguished from known teachings, the inventors have devised the present invention on the basis of a novel idea of maintaining the space between adjacent silicon steel sheets in a non-reducing atmosphere during the temperature rise of the box annealing treatment.

That is, a substance adapted to be gradually dehydrated at a temperature from 400to 830 C owing to it thermal decomposition which begins at the end of release of the bound water from magnesia is added as an ingredient of the separator to the magnesia so as to maintain the space between adjacent silicon steel sheets in the'non-reducing atmosphere. Thus, the surface of the silicon steel sheet remains in its oxidized state until the silicon in the steel sheet reaches its oxidation temperature whereby to obtain an excellent film closely adhered to the surface of the silicon steel sheet in an easy manner. I

The invention makes use of a particularly selected separator, which is capable of adjusting the annealing atmosphere in the space between adjacent silicon steel sheets and provides an improved method of forming a uniform electric insulating coating having excellent adherent and heat resisting properties on the surface of a wide silicon steel sheet. The method according to the invention may be applicable in case of high degree of hydration of magnesia, i.e., in case of a high content of bound water or even the case in which the bound water may tend to increase.

As explained hereinbefore, the use of the particularly selected separator compositions ensures formation of a coating film having excellent properties without requiring any particular adjusting operation of the annealing atmosphere and expensive solvents and further provides the important advantage that the degree of hydration of magnesia has no substantial effect upon the formation of the film with the result that the surplus of the separator remaining in the coating step can be used again The most important feature of the invention lies in the use of a separator which contains serpentine.

In general serpentine is an ore formed from peridotite by subjecting it to metamorphism and contains as its main ingredient antigorite and/or chrysotile and further contains as its impurities, normally present, talc, magnesite (MgCO dolomite (CaCO MgCO and chlorite (Mg ,,Al,,) (Al,,Si 0, (OI-I and iron oxides 1 60,.

The antigorite and chrysotile are layered structure magnesium silicates in the form of water bearing strata and are represented by the following chemical formula 3'lVIgO 2S1 ITI-TQOT The antigorite and chrysotile when heated at temperatures above about 200C are gradually dehydrated to decrease the weight thereof by about l3.0 percent.

Asrnentioned above, the serpentine ores preferred for the present invention are antigorite and or chrysotile. The term serpentine as used herein shall be understood to denote such ores or chemical equivalents thereof which gradually decompose or dehydrates over the temperature range of 200to 830C.

DETAILED DESCRIPTION The invention will now be described with reference to the annexed drawings, in which FIGS. la to 4a are curves illustrating the results of the differential thermal analysis subjected to the separator compositions to Example 1 to 4 respectively illustrating to 4b are curves illustrating the results of the thermo-gravimetric analysis subjected to the separator compositions according to said Examples illustrating the invention;

FIG. 5 shows the curves illustrating the relation of iron loss value before and after the stress relief annealing step of the silicon steel sheet treated by the invention when compared with that of the conventional silicon steel sheet; and

FIG. 6 shows the curves illustrating the magnetostriction of the silicon steel sheet treated by. the invention when compared with that of the conventional silicon steel sheet.

Referring now to FIGS. 1a, 1b to 4a, 4b, there are shown curves illustrating the results of differential thermal analysis*(*Differential Thermal Analysis, Theory and Practice, 1958, pages 36 to 39 Chemical Publishing Co., Inc., New York, and Duvals Inorganic Thermogravimetric Analysis, 1963, pages 1 and 2, Elsevier Publishing Co., New York. and thermogravimetric analysis subjected to the above mentioned separator composition of the respective appended examples.

In the differential thermal analysis curves shown in FIGS. 1a to 4a, an emdothermic peak at 380C shows that of magnesium hydroxide, endothermic peaks at 430C, 700C and 770C and exothermic peaks at 730C and 830C show those of the serpentine material, endothermic peaks at 610C and 800C show those of dolomite and an endothermic peak at 950C shows that of talc.

The thermogravimetric analysis curves shown in FIGS. 1b to 4b illustrate that the above mentioned composition of the Examples gradually decompose and de hydrate at a temperature range of 200C to 830C.

Thus, in case of boxannealing the serpentine added as an ingredient of the separator according to the invention, the thermal decomposition of magnesium hydroxide during the heating step terminates at about 400C and then the thermal decomposition of the serpentine produces water vapour and carbon dixoide with the result that the space between adjacent silicon steel sheets is maintained in moderate non-reducing atmosphere. 1

Experimental tests on the growth of the electrical insulating coating have yielded the surprising result that sole addition of the serpentine material to magnesia produces an excellent film, that the addition of a serpentine material containing certain amount of dolomite and talc to magnesia produces a more excellent film, and that sole addition of tack or dolomite to magnesia produces a very poor film.

The magnesium hydroxide in the coated separator decomposes during the heating to produce water vapour which causes an oxidizing atmosphere between adjacent silicon steel sheets thus oxidizing the surface of the silicon steel sheet. The surface of the silicon steel sheet thus oxidized is not reduced owing to the presence of the water vapour generated from the serpentine material up to a temperature of 820C to 950C. This iron oxide is reduced by the silicon in the silicon steel sheet at temperatures above the temperature at which silicon is oxidized and closely adhered to the substrate iron thereby forming a strong film.

It has heretofore been proposed to adjust the atmosphere in a furnace by blowing water vapour thereinto. But, such exterior control means is not capable of introducing wet atmosphere into the space between adjacent silicon steel sheets surfaces of the wound sheet or of the laminated sheet in a uniform manner. Moreover, it is difficult to difiuse the wet atmosphere into the space between adjacent silicon steel sheets with the aid of such exterior control means.

There are a number of compounds containing bound water. But, the serpentine mineral is the most preferable compound since its dehydration occurs successively after the thermal decomposition of magnesium hydroxide and is continued up to the temperature at which silicon is oxidized.

The serpentine material, after removing its impurities, decomposes when heated to make silicon dioxide and free water and is converted into forsterite which is the same as the main ingredient of the film in accordance with the following formula, 2(3MgO-2SiO -2- H O) 3(2MgO-SiO SiO 4H O (1) This property of the serpentine material is suitable for the separator.

As above mentioned, natural serpentine ores contains as normal impurities chlorite, magnesite, dolomite iron oxide and talc, etc. In order to attain the object of the invention the presence of less than 2.3 percent by weight of alumina, less than 6.2 percent by weight of calcium oxide and less than 5.4 percent by weght of iron oxide is satisfactory to form normal films. In accor dance with the invention, if the amount of carbonate such as magnesite or dolomite is less than 25 percent by weight as MgCO there is no problem at all. Such an impurity acts to carburize the steel sheet when it is annealed so that it has been the common practice to avoid the use of such impurity. On the contrary, separator according to this invention is capable of maintaining the partial pressure of water vapour between the steel sheets with the aid of the water vapour produced from the serpentine and of preventing the reduction of CO produced when the carbonates are decomposed to CO, and as a result, the presence of impurity as MgCO 3 in the amount less than 25 percent by weight is allowable in carrying out the invention.

The serpentine containing, as its impurities dolomite and talc is capable of forming an excellent film. But, when dolomite or talc is used instead of serpentine a poor film is produced.

As above mentioned the invention allows a comparatively large amount of carbonates such as magnesite or dolomite to be used. Because, the partial pressure of the water vapour existing in the space between the adjacent silicon steel sheets is maintained high so that a carburizing atmosphere is not produced, even if carbon dioxide is generated.

The carbon dioxide serves to maintain iron oxide on the surface of the silicon steel sheet in non-reduced state so that generation of carbon dioxide is rather desirable.

It is preferable that the separator coating composition contains 5 percent to 40 percent by weight of the serpentine material so that the serpentine material in its dried state will be present in an amount of 0.5 g/m on the sheet to 6.0 g/m The serpentine material affords no substantial adverse effect upon the use of the separator even when the separator layer contains an extremely small amount of the bound water of magnesia so that substantially no oxidation occurs on the surface of the silicon steel sheet during annealing whereby to form an excellent coating.

Another feature of the invention consists in the use of manganous oxide (MnO) powder as another ingredient of the separator which contains the above mentioned magnesia and serpentine.

The manganous oxide mixed with the magnesia and serpentine material powders is not reduced by hydrogen in the atmosphere during the box annealing treatment, but can selectively oxidize silicon in steel at temperature above about 800C to deposite a silicon dioxide film on the surface thereof while being reduced into manganese, substantially all of which is diffused into the steel.

The above mentioned action of the manganous oxide powder is believed due to such reasons that the standard free energy of producing manganous oxide at a temperature of 600to 1,400C is smaller than that of silicon dioxide, but is larger than that of iron oxide and water, and that the crystal configuration of manganous oxide, which is a cubic system is the same as that of magnesia so that manganous oxide and magnesia solid solute each other.

Manganous oxide can solid solute into magnesia so that that portion of the manganous oxide particles which is not in contact with the surface of the steel sheet can diffuse through the magnesia particles into the surface of the steel sheet and hence is reduced by the silicon in the steel sheet.

Manganous oxide as such is not available on the market, so that use may be made of those oxides, hydroxide, carbonates or oxalates, etc. of manganese which when heated or subjected to a reducing treatment can be changed into manganous oxide. It is preferable to heat the above mentioned compounds in a reducing atmosphere containing hydrogen.

But, it is not preferable to'use these compounds without previously reducing them into manganous oxide as these compounds, when subjected to a temperature rise during the box annealing treatment, are decomposed to generate oxygen gas.

The box annealing treatment should be carried out in a reducing atmosphere containing hydrogen. In accordance with the invention substantially no hydrogen etc. penetrates into the space formed between adjacent steel sheets in the form of a coiled or laminated body at temperature up to at least 820C owing to the presence of the water vapour produced from the serpentine material. Thus, manganous oxide is not reduced to produce water vapour, but is liable to be heat decomposed thereby generating oxygen gas.

Nascent oxygen occluded in the water vapour acts to excessively oxidize the surface of the steel sheet with the result that a thin uniform film having an excellent adherent property could not be formed on the surface of the steel sheet.

For example. if manganese dioxide (M110 is heated in hydrogen its reduction terminates at about 480C to produce manganous oxide.) But, if manganese dioxide is heated in a non-reducing atmosphere, oxygen is generated to change the manganese dioxide through manganic oxide (Mnto manganomanganic oxide s 4)- Experimental tests have yielded the result that the use of manganous oxide containing compositions containing the other oxides of manganese in an amount such that manganese dioxides less than 3 percent, manganic oxide is less than percent and manganomanganic oxide is less than 25 percent ensures formation of a film having excellent properties which is substantially the same as that obtained by using pure manganous oxide.

But, it is not preferable to use sulfates, chlorides or nitrates of manganese since these compounds generate gases of sulphur dioxide (80,) hydrogen chloride (l-ICl) and mononitrogen dioxide NC etc. The method according to the invention, is capable of controlling the amount of silicon dioxide formed on the surface of the silicon steel sheet with the aid of the amount of Mn 0 in the separator.

In the invention the effective amount of manganous oxide to be coated on the silicon steel sheet in the composition according to this invention is limited in its dried state to the range of from 0.5 g/m to 6 g/m. If the amount of manganous oxide is more than 6 g/m too much amount of manganese is reduced from the manganous oxide and the manganese thus reduced is diffused into the silicon steel sheet thus deteriorating the magnetic properties thereof.

It is preferable to limit the manganous oxide in its dried state to 1.5 g/m to 5 g/m in order to form a film having excellent properties. The separator coating composition should contain 4 percent to 40 percent by weight of manganous oxide in order to coat the silicon steel sheet with the above mentioned amount.

The amount of manganous oxide to be coated may slightly be decreased if silicon dioxide is already present on the surface of the silicon steel sheet Thus, the invention is capable of forming a glass film on the surface of a silicon steel sheet in a stable manner notwithstanding the presence or absence of silicon dioxide produced on the surface of the silicon steel sheet owing to a continuous open annealing treatment prior to the coating of the separator composition according to this invention. It is desirable to make the thickness of the silicon dioxide film produced before and after the high temperature box annealing treatment, less than 2.5 ,u..

As the main ingredient of the separator containing serpentine material added with or without manganous oxide, use may be made of magnesia such as light magnesia available in market. A pair of such magnesia may be substituted by heavy magnesia or inactive magnesia adapted to be calcined at a higher temperature.

In the invention the amount of magnesia to be coated is limited in its dried state to 2 g/m to 12 g/m.

If less than 2 g/m of magnesia is coated on the surface of the silicon steel sheet, impurities in the silicon steel sheet could not sufficiently be removed thus a silicon steel sheet having desired magnetic properties can hardly be obtained.

If more than 12 g/m of magnesia is coated on the surface of the silicon steel sheet, it takes a lot of time to dry the separator coat from the slurry but also to cause a temperature rise owing to the heat insulating effect of magnesia. Moreover, the temperature distribution in the silicon steel sheet in the form of a coiled or laminated body becomes non-uniform.

The separator according to the invention may be coated on the surface of a silicon steel sheet without necessitating any special method. For this purpose, use may be made of any conventional method of coating the separator on the surface of the electrical silicon steel sheet in a substantially uniform manner. That is, the separator in the form of an aqueous slurry may be coated on the surface of the electrical silicon steel sheet and the steel sheet thus coated may be passed through rubber rolls to remove any undesired excessive amount of the separator slurry and then dried. The excessive amount of the slurry removed by the rubber rolls etc. may be collected into its supply tank and then recycled.

The silicon steel sheet coated with the dried separator composition according to the process of this invention is then box annealed at a temperature of l,l00to 1,300C for more than 2 hours to form an electric insulating film on the surface of the silicon steel sheet.

As the annealing atmosphere, it is most preferable to use a reducing atmosphere containing hydrogen, but nitrogen may also be added to it during the temperature rise up to about 400C.

Although it is not always necessary to use the reducing atmosphere in its dried state, it is preferable to use a hydrogen atmosphere having a dew point of less than -30C at a temperature above l,000C.

The coating formed by the method according to the invention has an excellent electric insulating property whose interlaminar resistance has values above 10 Q-cm llayer.

Experimental tests of bending the steel sheet coated with the above mentioned coating around a round bar having a diameter of 5 mm by l yielding the surprising result that substantially no flake off of the film from the steel sheet occurs.

The thickness of the film may be made on the order of 2p. to 4;. so that the space factor thereof becomes very high.

A stress relief annealing treatment in a nitrogen atmosphere at 800C for 5 hours subjected to the film showed that substantially no change in its various properties occurs.

Adhesion strength tests of the film with the aid of epoxy resin also showed that the adhesive force is above 200 Kg/cm if a treating agent such as phosphate etc. commonly used in non-oriented electrical silicon steel sheet is coated in the form of a thin layer on the film and baked to the latter, the electric insulating property of the film can be improved without deteriorating its adherent property and lamination factor as defined by ASTMA- 344.

The invention will now be to examples.

described with reference EXAMPLE 1 A cold rolled silicon steel sheet containing 3.15 percent by weight of silicon and 0.06 percent by weight of manganese and having a thickness of 0.30 mm was subjected to a continuous open annealing treatment in an atmosphere consisting 65 percent by volume of hydrogen and 35 percent by volume of nitrogen and having a dew point of +20 "C at 820C for 4 minutes. 60 Kg of the mixed powder consisting of 30 percent by weight of natural serpentine material and the remainder of light magnesia powder was suspended in 1,000 I of water to produce an aqueous suspension coating material adapted to be used as a separator. This separator slurry was coated on the surface of the above mentioned steel sheet and the steel sheet thus coated was dried at 200C for 1 minute. The dried steel sheet was then wound into a coiled body. respectively. The amount of the coated and dried separator was 12 g/m whose differential thermal analysis and thermogravimetric analysis curves are shown in FIGS. 1a and 1b, respectively. The amount of hydration of magnesia was 11 percent.

The coiled body was then box annealed in a hydrogen stream at 1,150C for hours. The box annealed coiled body was then cooled and the unreacted separator was removed. Observation of the surface of the steel sheet showed that a glass film is uniformly formed throughout the total surface thereof.

Experimental tests of bending the steel sheet treated as above described around a round bar having a diameter of 5 mm by 180 yielded the result that substantially no flake off of the film from the steel sheet occurs.

The interlaminar electrical resistance of the steel sheet coated with the above mentioned electric insulating film was 12.7 fl-cm llayer.

Stress relief annealing treatment subjected to the above treated steel sheet in a nitrogen atmosphere at 820C for 5 hours showed that substantially no change in its adherent property occurs and that the interlaminar resistance of the steel sheet becomes 31.6 (Lem llayer. Adhesion strength test of the film with the aid of epoxy resin also showed that the film is not taken off the steel sheet until 220 kg/cm EXAMPLE 2 A silicon steel having the same composition as that described in the example 1 was cold rolled into a sheet having a thickness of 0.3 mm. A mixture consisting of 30 percent by weight of natural serpentine material containing about 17 percent by weight of dolomite, 20 percent by weight of manganous oxide and the remainder of light magnesia power of 325 Tyler meshes is used. 50 kg of the mixed powder was suspended in 1,000 l of water to form an aqueous suspension coating material to be used as a separator. This separator was coated on the surface of the steel sheet and the steel sheet thus coated was dried under the same condition as that described in the example 1. The dried sheet was then wound into a coiled body.

The amount of the coated and dried separator was 20 g/m whose differential thermal analysis and thermogravimetric analysis curves are shown in FIGS. 2a and 2b, respectively. The amount of hydration of magnesia was The coiled body was then box annealed in a hydrogen atmosphere at 1,150C for 10 hours. The box annealed coiled body was then cooled and the unreacted separator was removed. Observation of the surface of the steel sheet showed that a grayish glass film is uniformly formed throughout the total surface thereof. The amount of manganese in the steel after being subjected -to the annealing treatment was 0.18 percent by weight.

Experimental tests of bending a test piece cut from the above treated steel sheet yielded the result that sub; stantially no crack occurs in the film even after the tests repeated for 21 times. Moreover, experimental tests of bending the test piece around a round bar having a diameter of 5 mm by 180 yielded the result that substantially no flake off of the film from the steel sheet oc- Cuts.

The interlaminar electrical resistance of the silicon steel sheet coated with the insulating film was 15.7 fl-cm llayer.

EXAMPLE 3 A silicon steel having the same composition as that described in the example 1 after being cold rolled to a thickness of 0.35 mm, was subjected to a continuous open annealing treatment in a reducing atmosphere consisting of percent by volume of hydrogen and 30 percent by volume of nitrogen and having a dew point of +5C at 880C for 5 minutes. A mixture consisting of 20 percent by weight of natural serpentine material The amount of the coated and dried separator was 17 g/m whose differential thermal analysis and thermogravimetric analysis curves are shown in FIGS. 3a and 3b, respectively. The rate of hydration of magnesia was 8 percent.

The coiled body was then box annealed in a hydrogen atmosphere at 1,200C for 5 hours.

The box annealed coiled body was then cooled and the unreacted separator was removed. Observation of the surface of the steel sheet showed that a grayish glass film is uniformly formed throughout the total surface thereof.

Experimental tests of bending the steel sheet treated as above described around a round bar having a diameter of 5 mm by yielded the result that substantially no flake off of the film from the steel sheet occurs.

The interlaminar electrical resistance of the silicon steel sheet coated with the electric insulating film was 25.1 Q-cm /laye r.

EXAMPLE 4 A silicon steel having the same composition as that described in the example'l after being cold rolled to a steel sheet having a thickness of 0.28 mm was subjected to a continuous open annealing treatment in a reducing atmosphere consisting of 60 percent by volume of hydrogen and 40 percent by volume of nitrogen and having a dew point of +50C at 850C for 5 minutes. A mixture consisting of 24 percent by weight of natural coating material adapted to be used as a separator. This 1 separator was coated on the surface of the above mentioned steel sheet and the steel sheet thus coated was dried at 300C for 1 minute in a continuous manner. The dried steel sheet was then wound into a coiled body.

The amount of the coated and dried separator was g/m whose differential thermal analysis and thermogravimetric analysis curves are shown in FIGS 4a and 4b, respectively. The rate of hydration of magnesia was 9 percent.

A coating reagent obtained by solving 6 kg of chromic anhydride and kg of aluminum nitrate in 1,000 l of 30% aqueous magnesium phosphate was coated on the surface of the above mentioned film and the film thus coated was subjected to a baking treatment at 450C for 1 minute. The amount of the coated and baked coating reagent was 2.5 g/m and the interlaminar electrical resistance of the steel sheet coated with the above mentioned film was improved to 75 Q-cm /layer. In this case, there occurred substantially no change of the adhesive property of the above mentioned film.

" A stress relief annealing treatment in a nitrogen atmosphere at 800C for 5 hours subjected to the film showed that the interlaminar resistance is further improved to 130 (t-cm llayer. Adhesive strength tests of the film with the aid of epoxy resin showed that the adhesion strength is 250 kg/cm Ftp armament have stamina? the cleansers; lating coating formed on the surface of the silicon steel sheet is excellent in properties thereof and that the silicon steel sheet is not liable to be deteriorated in iron loss by cold working and excellent in a magnetostriction property.

Table 1 shows the difference between the 'w g;

values and the difference between B values of both of the Epstein test pieces measured before and after the stress relief annealing treatment, respectively, and also shows results of recovery obtained by the stress relief annealing treatments.

is/so m) 10 m) Mnxi- Mini- Aver- Mnxi- Miul- Aver- Trenting method mum mum ego mum mum inn Method according to the invention:

Dlflerenec 0. l1 0. 03 0. 07 0. 4 0. 1 0. 3 Rate of recovery,

percent 10.0 2. l) 6. 9 2. 3 0. 0 l. 7 Conventional method:

Difference 0. 32 0. l6 0. 24 0. 6 0. 2 0. 4 Rate of recovery,

percent 21'. 8 13. 6 19. 6 3. 5 l. 1 2. 3

NOTE 1.Difierence=(value before the stress relief annealing treatment)-(value after the stress relief annealing treatment).

NOTE 2.Rate 0t recovery=(the difference/the value before the stress relief annealing treatment) X(%).

7x; seen from FIG. 5 and the ta blei, the method ac cording to the invention can fabricate grain oriented silicon steel strips whose rate of recovery of magnetic pEs mes' 'saiEwnH' was after the Stress relief annealing treatment are far smaller than those of the grain oriented silicon steel strips obtained by the conventional method and further provides the important advantage that a cold working treatment such as shearing treatment can be effected to the steel sheet without deteriorating the iron loss thereof with the result that it is not always necessary to apply the stress relief annealing treatment to the steel strips. Further, the grain oriented silicon steel sheet treated by the method according to the invention is less sensitive to the magnetostriction effect.

FIG. 6 shows the results of magnetostriction in lengthwise direction of grain oriented silicon steel sheets treated by the method according to the invention and also treated by the conventional method which solely makes use of magnesia as a separator, the results are obtained by a strain gauge.

As seen from a curve b in FIG. 6, the steel sheet treated by the conventional method remarkably increases its magnetostriction in dependence with the increase of the magnetic flux density thereof, while the steel sheet treated by the method according to the invention has extremely small magnetostriction values up to the magnetic flux density of about 19 KG. Experimentation has shown that substantially no change in the above mentioned magnetostriction values occurs after stress relief annealing.

The reason why the steel sheet treated by the method according to the invention has extremely small magnetostrictive values is due to the fact that the steel sheet thus treated can be subjected to machining works without deteriorating the magnetic properties thereof owing to the presence of the film formed thereon by the method according to the invention. This is proved by v the experimental result that if the steel sheet treated by the method according to the invention is subjected to a pickling treatment, etc. to remove the film formed thereon the naked steel sheet changes its magnetic properties when subjected to a shearing treatment and its magnetostrictive values when subjected to a change in its state of magnetization in entirely the same manner as is effected by the steel sheet treated by the conventional method.

It will be obvious that the invention is not restricted to the examples described and that many variations are possible to those skilled in the art without departing from the scope of this invention.

What is claimed is:

1. In a method of forming an electrical insulating coating on the surface of a silicon-steel sheet by coating the surface of said silicon-steel sheet containing 2 to 4 percent weight of silicon with a separator composition in a aqueous suspension state, drying the coated silicon steel sheet, and box-annealing the coated and dried sheet in a reducing atmosphere containing hydrogen, the improvement wherein said separator composition contains from S to 40 percent by weight of serpentine, from to 4 percent by weight of manganous oxide, the remainder of magnesia, said serpentine in said coating in its dried state, containing 0.5 g/m to 6.0 g/m of serpentine and 2.0 g/m to 12 g/m of magnesia and said coated separator composition having the property of gradually decomposing and dehydrating over a temperature range of from about 200to 830C.

2. A metliod as claimed in claim 1. wherein said separator composition consists essentially of to 40 percent by weight of serpentine material, 4 to 40 percent by weight of manganous oxide and the remainder of magnesia and is coated on said silicon steel sheet in amounts so that said composition in its dried stated will contain, 0.5 g/m 6.0 g/m of manganous oxide and 2 to l2 g/m of magnesia.

3. A method as claimed in claim 1 wherein said reducing atmosphere containing hydrogen further contains nitrogen.

4. A method as claimed in claim 1 wherein said serpentine material is a natural mineral that has its main ingredient antigorite or chrysotile (3Mg0' 2SiO,- 2H O) and further may include normally co-present talc, magnesite (MgCO dolomite (CaCO MgCo and chlo rite and iron oxide in amounts providing less than 2.3 weight percent of alumina, less than 6.2 weight percent of calcium oxide and less than 5.4 weight percent of iron oxide.

5. A method as claimed in claim 1 wherein said sili: con steel sheet is subjected to a continuous open annealing treatment before it is coated with said aqueous separator composition. 

2. A method as claimed in claim
 1. wherein said separator composition consists essentially of 5 to 40 percent by weight of serpentine material, 4 to 40 percent by weight of manganous oxide and the remainder of magnesia and is coated on said silicon steel sheet in amounts so that said composition in its dried stated will contain, 0.5 g/m2 6.0 g/m2 of manganous oxide and 2 to 12 g/m2 of magnesia.
 3. A method as claimed in claim 1 wherein said reducing atmosphere containing hydrogen further contains nitrogen.
 4. A method as claimed in claim 1 wherein said serpentine material is a natural mineral that has its main ingredient antigorite or chrysotile (3MgO. 2SiO2. 2H2O) and further may include normally co-present talc, magnesite (MgCO3), dolomite (CaCO3. MgCO3) and chlorite and iron oxide in amounts providing less than 2.3 weight percent of alumina, less than 6.2 weight percent of calcium oxide and less than 5.4 weight percent of iron oxide.
 5. A method as claimed in claim 1 wherein said silicon steel sheet is subjected to a continuous open annealing treatment before it is coated with said aqueous separator composition. 